Port some displayWidth/displayHeight usages to Screens::size()

Summary:
KWin::displayWidth and KWin::displayHeight are bound to X11 which
doesn't make much sense on X11. In addition KWin internally knows
the overall display dimensions through the Screens singleton class.

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D1798